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the moving component of a reciprocating engine that is fitted to the internal surface of a cylinder and moves back and forth along the direction of the cylinder’s axis. In engines, power cylinders, and presses, the piston transmits the pressure of a working fluid—gas, vapor, or liquid—to the moving parts. In some types of engines, such as two-cycle internal-combustion engines, the piston also plays a role in the gas distribution process. In pumps and compressors, the suction, compression, and delivery of the liquid or gas are accomplished by the reciprocating piston.
A piston may be of the trunk, disk, or plunger type, depending on the piston’s length-to-diameter ratio and on the piston’s design. The trunk piston, whose length is somewhat greater than its diameter, has a head, grooves for piston rings, and a guide skirt. The height of a disk piston is determined only by the size of the sealing device; the rod on which the piston is mounted serves to align the piston. The plunger piston, whether a plunger, ram, or pin, usually operates with a smooth surface; its length is several times greater than its diameter.
In rotary-piston internal-combustion engines, a rotor performs the functions of a piston in transmitting the pressure of a working fluid to the moving parts.